1. Field of the Invention
The invention relates to an upflow/downflow high efficiency furnace for providing heated air to a confined space; and a method for mounting a secondary heat exchanger in a furnace, to permit the furnace to be installed for either an updraft or downdraft mode of operation.
2. Description of the Prior Art
Due to the cost and shortage of natural gas, attempts have been made to design and construct more efficient natural gas-fired hot air furnaces. One method for maximizing the heat energy transferred from the heating fluid, or combustion gases, to the air to be heated, or the air in the enclosure or space to be heated, is to transfer as much latent heat as possible from the water vapor in the heating fluid, or combustion gases, to the air to be heated. Thus, increases in furnace heating efficiency have been accomplished by cooling the flue combustion gases of the heating fluid, while still within the furnace, to below the dew point to recover some of the latent heat of vaporization as usable energy. This is generally accomplished by adding a secondary condensing heat exchanger to the primary heat exchanger, and passing air to be heated initially over the condensing secondary heat exchanger, and then over the primary heat exchanger. Depending on the type of condensing furnace, efficiencies can be in the low to mid 90% range.
In such prior art furnaces, a major disadvantage is that a single furnace design may not be utilized for both updraft and downdraft modes of operation for the furnace. An updraft furnace is a furnace wherein a blower, adapted to blow air over the primary and secondary heat exchangers, is disposed beneath the primary and secondary heat exchangers, and blows the air upwardly over the heat exchangers. A downdraft furnace is one wherein the blower is mounted above the primary and secondary heat exchangers, and blows air downwardly over the heat exchangers. Prior art furnaces are designed only for operation in either an upflow or downflow mode, whereby a heating contractor would have to maintain two different models of furnaces in his or her inventory, in order to be able to install both upflow and downflow furnaces. From a manufacturing standpoint, it is also necessary to have two differently designed furnaces, having different parts therein, in order to supply customer needs for both upflow and downflow furnaces. Accordingly, manufacturing, storage, and inventory costs are increased in order to provide both types of furnaces.
Another disadvantage associated with high efficiency furnaces utilizing a secondary, or condensing, heat exchanger is that because of the temperature of the combustion gas being lowered within the secondary heat exchanger to a temperature below its dew point, the water vapor in the combustion gas condenses and moisture can form in the secondary heat exchanger. The moisture, in combination with other byproducts found in the combustion gases, can be quite corrosive and can freeze, whereby it is necessary to ensure that the moisture and combustion by-products, or collectively "condensation", do not collect and remain in an appreciable amount within the secondary heat exchanger. Should such moisture and combustion gas by-products, or condensation, not be removed from the secondary heat exchanger, they can corrode the metal surfaces of the secondary heat exchanger, or block the fluid passageways through the secondary heat exchanger. This could create a leak path for the combustion gases, which can present a serious health hazard, should such combustion gases leak into the space, or room, being heated or cause a pressure build-up which would turn off the furnace. If the secondary heat exchanger is disposed above the primary heat exchanger, such condensation if it passes downwardly into the primary heat exchanger could also cause damage to the primary heat exchanger and perhaps cause its premature failure. Accordingly, prior art high efficiency furnaces typically require many extra components in order to provide for the removal of the moisture and combustion gas by-products or condensation, from the secondary heat exchanger. These additional parts increase the cost of manufacturing of the furnace.
Accordingly, prior to the development of the present invention, there has been no high efficiency furnace which: can be installed for either an upflow or downflow mode of operation; is easily, efficiently, and economically manufactured; and efficiently prevents moisture and combustion gas by-product remaining in the secondary heat exchanger and blocking the passage of combustion gases through the secondary heat exchanger. Therefore, the art has sought a high efficiency gas furnace which: can be installed for either an updraft or downdraft mode of operation; is easily, efficiently, and economically manufactured; and prevents moisture and combustion gas by-products remaining in the secondary heat exchanger and blocking the passage of combustion gases therethrough, or prevents such moisture and combustion gas by-products flowing back into the primary heat exchanger.